This invention relates to a pinion-rack drive system, and especially to an auxiliary lock mechanism incorporable into the system to retain the movable element of the system in a locked condition when the pinion is inactive as a drive component. The invention is believed applicable in various fields where pinion-rack systems are used, e.g. vehicle seat adjusters, turret drive systems in military tanks, gun elevation adjusting systems in tanks or stationary platforms, lathes or similar machine tools, and jacks or hoists.